1. Field of the Invention
The present invention relates in general to the fabrication of an integrated circuit (IC) device. More particularly, the present invention relates to a method of forming a shallow trench isolation (STI) structure.
2. Description of the Related Art
Integrated circuit (IC) usually contains thousands of metal-oxide semiconductor field effect transistors (MOSFET) or devices. As integration increases, isolation structures are now of great importance for isolating individual devices in high integration integrated circuits. Inferior isolation structures may cause short circuit between adjacent transistors. One of the most common used isolation structures is local oxidation (LOCOS), which is now a mature process with high reliability and efficiency. However, there are still several drawbacks of LOCOS, including stress, consequential problems due to stress and the formation of bird's beak. In particular, the formation of bird's beaks hinders development of high integration devices, especially for sub-quarter-micrometer technology. Therefore, another common isolation structure, shallow trench isolation (STI) structure, becomes more popular in high integration devices. Furthermore, STI technique can provide a global planar surface of the isolation structure, useful for the subsequent processes.
For the conventional STI fabrication processes, trenches are anisotropically etched into the silicon substrate by photolithography and dry etching and a silicon oxide layer is deposited to fill the trenches. Afterwards, chemical mechanical polishing (CMP) technology is used to planarize the oxide layer so as to form oxide plugs in the trenches. However, many challenges are encountered in the design of the STI process. Especially, the mechanical stress induced during the STI process has deleterious influences on the device characteristics. Usually, during the etching step or the following cleaning processes, the edges of the oxide plugs may be eroded, thus forming divot(s) between the top corners of the trenches and the oxide plugs. The divot tends to accumulate charges and consequently induces sub-threshold leakage current, which is so-called kink effect. The undesired kink effect reduces the isolation ability and the quality of the devices and also reduces the yield.